A comparative study of the performance of a trickle bed reactor (TBR) and a monolith cocurrent downflow contactor (CDC) reactor in terms of selectivity and residence time distribution was conducted for the hydrogenation of 2-butyne-1,4-diol (13). Selectivity (S) towards 2-butene-1,4-diol was investigated with the solvent 2-propanol and a 30% (v/v) 2-propanol/water mixture (M) in batch recycle mode. Liquid residence time distribution (RTD) curves were obtained for both reactors. Although both reactors presented almost identical hydrodynamic behaviour, i.e. RTD, significant differences regarding selectivity towards the alkene were observed in both solvents. The use of 2-propanol gave lower selectivities in both reactors, but even then the monolith reactor was superior. In the monolith CDC, the liquid RTD curve was also obtained at different radial positions. RTD profiles across the monolith showed that from the centre to the column wall there is possibly an increased retention of material and despite this, overall selectivity does not appear to be considerably depressed by the backmixing that the above result implies in 2-propanol/water where the selectivity was found to be 100% towards the intermediate (C). Modelling of the monolith CDC reactor was also conducted to predict RTD. The models tested were tanks-in-series, piston exchange and piston dispersion exchange; from which, piston exchange model was found to best predict and fit the experimental data. (c) 2005 Elsevier B.V. All rights reserved.